Wednesday, January 30, 2008

Earlier today, an asteroid as wide as a football field cruised past Mars at a distance just over three times the diameter of the red planet. By cosmic standards, this was a very near miss. The asteroid was discovered in late November, and in mid-December astronomers placed the odds of an impact at 1-in-75. Additional observations just before the new year increased these odds to nearly 1-in-25, but within two weeks the probability of an impact sank dramatically to just 1-in-10,000. All of the attention given to the developing story led a satire website to release an article headlined: "NASA plans to blow up Mars if asteroid misses".

With such wildly oscillating predictions, it would be easy to imagine that scientists simply don't know what they're doing. But predicting the path of an asteroid in space is a little like predicting the path of a hurricane over the ocean -- natural uncertainties in the observations that are used for the calculations lead to a range of possible positions as we predict further into the future. In the case of an asteroid, astronomers use the observed positions of the space rock over time to define an orbit from Kepler's laws. With positions observed over a reasonable fraction of the asteroid's path around the Sun, the orbit can be calculated very accurately. But with data spanning only a few weeks, the orbit is harder to define and each new observation can lead to significant changes when estimating the future path of the asteroid.

This is exactly what happened in the case of the Martian asteroid, officially named "2007WD5". While routine calculations of asteroid trajectories typically lead to one-in-a-million odds for any planet impacts, the early observations of 2007WD5 gave a better than 1% chance of striking Mars. This got the attention of many astronomers, and new observations came pouring in from around the globe, leading to a better estimate of the orbit and a new calculation of the impact probability, by then up to 4%. This led to even more excitement, and many new observations -- defining the orbit so well that an impact was effectively ruled out.

As cool as it would have been to watch a big rock punch a fresh crater into Mars, we should be comforted by the fact that astronomers are conducting their census of the asteroid belt. After all, the next near miss could belong to the Earth...